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       setenv - change or add an environment variable


       #include <stdlib.h>

       int setenv(const char *name, const char *value, int overwrite);

       int unsetenv(const char *name);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       setenv(), unsetenv():
           _POSIX_C_SOURCE >= 200112L
               || /* Glibc versions <= 2.19: */ _BSD_SOURCE


       The  setenv()  function adds the variable name to the environment with the value value, if
       name does not already exist.  If name does exist in the environment,  then  its  value  is
       changed  to value if overwrite is nonzero; if overwrite is zero, then the value of name is
       not changed (and setenv() returns a success status).  This function makes  copies  of  the
       strings pointed to by name and value (by contrast with putenv(3)).

       The  unsetenv() function deletes the variable name from the environment.  If name does not
       exist in the environment, then the function succeeds, and the environment is unchanged.


       setenv() and unsetenv() functions return zero on success, or -1 on error, with  errno  set
       to indicate the cause of the error.


       EINVAL name is NULL, points to a string of length 0, or contains an '=' character.

       ENOMEM Insufficient memory to add a new variable to the environment.


       For an explanation of the terms used in this section, see attributes(7).

       │InterfaceAttributeValue               │
       │setenv(), unsetenv() │ Thread safety │ MT-Unsafe const:env │


       POSIX.1-2001, POSIX.1-2008, 4.3BSD.


       POSIX.1 does not require setenv() or unsetenv() to be reentrant.

       Prior  to  glibc  2.2.2,  unsetenv()  was  prototyped as returning void; more recent glibc
       versions follow the POSIX.1-compliant prototype shown in the SYNOPSIS.


       POSIX.1 specifies that if name contains an '=' character, then setenv() should  fail  with
       the error EINVAL; however, versions of glibc before 2.3.4 allowed an '=' sign in name.


       clearenv(3), getenv(3), putenv(3), environ(7)


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